Investments - Fair Value Measurements of Cash Equivalents Held in Money Market Funds and Investments (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities	$ 283,945	$ 296,062
Deferred Compensation Plan Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities	16,572	16,294
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities	132,388	111,261
Municipal Fixed-Rate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities	93,117	127,787
Municipal Variable Rate Demand Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities	4,245	2,465
Fair Value, Measurements, Recurring [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash equivalents	15,225
Available-for-sale securities	283,945	296,062
Total	299,170	297,225
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash equivalents	3,228
Available-for-sale securities	54,195	54,549
Total	57,423	55,712
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash equivalents	11,997
Available-for-sale securities	229,750	241,513
Total	241,747	241,513
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash equivalents	3,228	1,163
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash equivalents	3,228	1,163
Fair Value, Measurements, Recurring [Member] | Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash equivalents	11,997
Fair Value, Measurements, Recurring [Member] | Commercial Paper [Member] | Significant Other Observable Inputs (Level 2) [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash equivalents	11,997
Fair Value, Measurements, Recurring [Member] | Deferred Compensation Plan Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities	16,572	16,294
Fair Value, Measurements, Recurring [Member] | Deferred Compensation Plan Assets [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities	16,572	16,294
Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities	132,388	111,261
Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member] | Significant Other Observable Inputs (Level 2) [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities	132,388	111,261
Fair Value, Measurements, Recurring [Member] | Municipal Fixed-Rate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities	93,117	127,787
Fair Value, Measurements, Recurring [Member] | Municipal Fixed-Rate Bonds [Member] | Significant Other Observable Inputs (Level 2) [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities	93,117	127,787
Fair Value, Measurements, Recurring [Member] | Municipal Variable Rate Demand Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities	4,245	2,465
Fair Value, Measurements, Recurring [Member] | Municipal Variable Rate Demand Notes [Member] | Significant Other Observable Inputs (Level 2) [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities	4,245	2,465
Fair Value, Measurements, Recurring [Member] | Marketable Equity Securities - Technology Industry [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities	9,034	9,661
Fair Value, Measurements, Recurring [Member] | Marketable Equity Securities - Technology Industry [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities	9,034	9,661
Fair Value, Measurements, Recurring [Member] | Marketable Equity Securities - Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities	28,589	28,594
Fair Value, Measurements, Recurring [Member] | Marketable Equity Securities - Other [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities	$ 28,589	$ 28,594